1. Technical Field
The present invention relates to electrophoretic displays, and particularly to improvements in electrophoretic displays for use as thin electronic paper in the field of display mounting.
2. Related Art
Featuring thinness, lightness, and low power consumption, flat displays such as liquid crystal display devices are widely used in various fields such as office automation equipment, information terminals, watches, and televisions. Liquid crystal display devices, such as cellular phones, are currently required to be thinner, lighter, and tougher.
Electrophoretic displays (EPDs), though inferior to liquid crystal display devices in terms of color display and video display, have attracted attention as thin, lightweight, low-power-consumption displays. EPDs, which have properties similar to those of paper, are under development as “electronic paper” to be read.
A microcapsule EPD, a typical example of electronic paper, includes microcapsules containing a solvent in which charged particles are dispersed and electrodes between which the microcapsules are held. An electric field generated by applying a voltage between the electrodes changes the distribution of the charged particles, thus providing a contrast. This display can advantageously maintain a displayed image without power consumption because the charged particles, attracted by electrostatic and intermolecular forces, remain near the electrodes after the voltage application is terminated. Another advantage is that the display is not angle-dependent, is eye-friendly like paper, and is easy to look at for a long time because the viewer views the colors of the charged particles and solvent themselves (i.e., reflected light) rather than transmitted light, as from liquid crystal displays, or self-emitted light, as from organic electroluminescent (EL) displays.
The features of an EPD, including thinness, lightness, and flexibility, result from the fact that most of the materials forming the display unit, called an electrophoretic sheet, are organic. This causes a problem, however, in that a further reduction in thickness and weight would impair the strength of the materials themselves, and the display would be vulnerable to an external impact and no longer durable for use as a commercial product.
An effective approach to solving the above problem is to apply an impact-damping material used for flat display devices, such as plasma displays, to an EPD to avoid damage to the display device. In fact, JP-A-2006-150755 has already proposed a structure including an impact layer and an impact-absorbing layer to damp an impact on a glass substrate included in an EPD.
A microcapsule EPD requires a thicker impact-damping layer because the microcapsules themselves have a lower impact resistance than the glass substrate. Although the use of a thicker impact-damping layer improves impact resistance, forming an impact-damping layer with a thickness of not less than 1 mm on a thin display like electronic paper would impair the original advantages of an EPD, such as thinness and lightness, thus seriously degrading its commercial value.
Another problem is that electronic paper, potentially used like paper, tends to suffer damage to the glass substrate by an impact resulting from its own weight upon falling. An effective approach to solving this problem is to reduce the total weight of the electronic paper by reducing the thickness of the glass substrate, a component of greatest specific gravity, and minimizing the thickness of the impact-damping layer on the display, thereby damping an impact upon falling. This approach, however, has a problem in that the thinner glass substrate has a lower strength and therefore a significantly lower resistance to bending stress and that the microcapsules would be damaged because the thinner impact-damping layer would not sufficiently absorb an impact.
As for the feature of viewability comparable to paper, an EPD is significantly advantageous for use under sunlight because of being a reflective display; the viewer views light reflected from the charged particles. A problem arises, however, in that the display would be degraded by temperature and humidity changes when used outdoors.